fix febus overlap issue

[d-chambers]

Description

The way the overlapping time samples were calculated in the febus file format were off. Moreover, sometimes the percentage overlap in the header was set, but the block rate, temporal sampling, and data shape would indicate there is no overlap. This PR fixes both issues by calculating the number of overlapping samples based on the temporal sampling, block rate, and data shape and ignoring the block overlap attribute which can be wrong.

I also ran into a precision issue with the datetime64 object. One file @jinwar gave me has a temporal sampling rate of 3.33333333e-5. When converted to a timedelta64 this is 33333ns. It then causes some issues in determining the exact end of a file. In this case, the endtime is off by 0.003 seconds for a 5 minute file. Hopefully we can just work around this for now, but we may need to think of a better solution for cases like this in the future.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes.
• documented the new feature with docstrings or appropriate doc page.
• included a test. See testing guidelines.
• your name has been added to the contributors page (docs/contributors.md).
• added the "ready_for_review" tag once the PR is ready to be reviewed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.84%. Comparing base (3d635c1) to head (48a21b7).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #449   +/-   ##
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  Coverage   99.84%   99.84%           
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  Files         114      114           
  Lines        9184     9191    +7     
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+ Hits         9170     9177    +7     
  Misses         14       14           

Flag	Coverage Δ
unittests	99.84% <100.00%> (+<0.01%)	⬆️

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